1. Field of the Invention
The present invention relates to data networking and in particular to prioritizing access to nodes contained in a data network.
2. Background Information
A data network is a geographically distributed collection of interconnected communication links and segments for transporting data between nodes, such as computers. The nodes typically transport the data over the network by exchanging discrete frames or to packets containing the data in accordance with various pre-defined protocols, such as the Transmission Control Protocol/Internet Protocol (TCP/IP) or the Internetwork Packet eXchange (IPX) protocol.
Many types of networks are available, with types ranging from local area networks (LANs) to wide area networks (WANs). LANs typically connect nodes, such as is personal computers and workstations, over dedicated private communications links located in the same general physical location, such as a building or a campus. WANs, on the other hand, typically connect large numbers of geographically dispersed nodes over long-distance communications links, such as common carrier telephone lines. The Internet is an example of a WAN that connects disparate networks throughout the world, providing global communication between nodes contained in various networks.
WANs often comprise a complex network containing many different intermediate network nodes, such as routers or switches. These nodes are interconnected to form the WAN and are often configured to perform various functions associated with forwarding traffic through the WAN. One function typically performed by an intermediate node is implementing a routing protocol, such as the Border Gateway Protocol (BGP) or the Open Shortest-Path First (OSPF) protocol. Routing protocols typically enable the exchange of routing information that may be used by the intermediate nodes to forward (route) traffic carried by the nodes through the data network from a source to a destination.
Some data networks contain nodes, such as server nodes, that are configured to provide various services to nodes, such as client nodes, coupled to the network. In a typical arrangement, a client node accesses a particular service by issuing requests to the server node providing the service. The server node receives the request, processes it, and to depending on the nature of the request may respond to the client with results. For example, a network may contain a server that provides a Domain Name System (DNS) service for resolving a fully qualified domain name (FQDN) to an IP address. In a typical arrangement, a client accesses the DNS service by issuing a message (request) to the DNS server wherein the request contains the FQDN that is to be resolved. The DNS server processes the request, which may include searching a database to locate an IP address associated with the FQDN. If an IP address is found, the server sends a response message to the client containing the IP address of the FQDN. Otherwise, if the FQDN cannot be resolved (i.e., no database entries are associated with the FQDN), the server sends a response message indicating the FQDN could not be resolved.
In order to handle a large number of requests for a particular service issued by e.g., a multitude of client nodes, a data network may employ many servers, wherein each server is configured to provide the requested service. In a typical arrangement, an “anycast” address is associated with the service and each server providing the service is configured with the anycast address. As used herein, an anycast address refers to a single address assigned to a plurality of nodes. Servers typically utilize an anycast address to enable access to their particular service, such as a DNS service, a dynamic host control protocol (DHCP) service, or a rendezvous point (RP) associated with a protocol independent multicasting sparse mode (PIM-SM) service. A client typically accesses the service by issuing one or more requests containing the anycast address as a destination address in each request. Intermediate nodes in the network forward the requests to the server configured with the anycast address that is typically located at the shortest path from the requesting client. The server acquires the requests and processes them accordingly, which may include responding to the client.
One advantage with the above-described arrangement is that a client node need only know the anycast address associated with the service in order to gain access to the service. Thus, the client node need not be configured with individual addresses for each of the servers providing the service in order to access the service. Another advantage with the above-described arrangement is that it provides for a high degree of availability of the service as “seen” by the clients. For example, if any server that receives the request provides access to the service, if a particular server becomes unavailable, another server providing the same service can “step in” and provide the service in a manner that is transparent to the client. Accordingly, the client sees a high degree of availability with regards to the service and need not take any further action on its part if a particular server becomes unavailable.
One disadvantage associated with the above-described arrangement is that if the service involves ensuring that information provided to the clients is coherent among the servers providing the service, special steps may need to be taken to ensure that the information is synchronized among the servers. For example, assume a first server and a second server are configured as described-above with an anycast address that is associated with a seat reservation service provided by the servers. Further, assume a first client accesses the service by issuing a request containing the anycast address and that the first server acquires the request and reserves a seat for the client. Now assume a second client accesses the service by issuing a request containing the anycast address and the second server acquires the request. In order to avoid having the second server reserve the same seat for the second client that was reserved for the first client, the second server must know the availability of the seat before it reserves a seat for the second client. One way this can be done is to have the second server synchronize its reservation information with first server before the second server reserves a seat for the second client.
Synchronizing information between servers may involve running a synchronization protocol on the servers that synchronizes the information among the servers. One problem with synchronization protocols is that they may be difficult to configure and may impact the performance of the servers, as the servers must dedicate resources to execute the protocol. Moreover, synchronization may affect client response time for various requests as information may have to be synchronized before a particular request can be completely processed. This, in turn, may act to further impact the server's response time to the client, as well as act to limit the server's capacity to handle requests.